Hi Guys,
I have the 6L diesel in my 2004, F250 super crew truck. Only 18,000 miles on it and am having intermittent problems with the a/c blowing air out the defrost vents. Sometimes it works and other times not. My question is: If I switch from 2 wheel drive to 4 wheel high and get the indicator light on the panel to light up, is my truck actually in 4 wheel drive? I get the indicator light all the time by the way. Does this mean my problem may be elsewhere? I hate to replace the vacuum pump if not needed. Thanks - Bob

 

The 4x4 light on your dash indicates that the transfer case has successfully shifted. That shift is done by an electric motor. No vacuum involved.

Vacuum is used to lock your automatic hubs, and there is no sensor or feedback to indicate whether they locked or not. In your case, you have a vacuum leak in the locking hub system, and that is why your air switches to defrost. There is insufficient vacuum to operate the vents due to the leak in the locking hub system. Usually it is the actual hubs that are leaking, but sometimes it is a broken vacuum line, or a shift valve, or even the pump itself. If you search for ESOF problems you will find some info on troubleshooting. Don't just buy a pump, because that probably is not the issue.

 

Check the vacuum solenoid that controls the hubs. It's located on the pass. side fender just behind the vacuum tank. Mine went bad and I had the exact same symptoms as you did. Just swapped the part out and problem solved. Relatively cheap part.

 

Thanks guys. I'll take a look for that vacuum solenoid.
Bob

 

Hi Guys,
I picked up a solenoid from Auto Zone today, but unfortunately, it's the wrong one. Their model had a two pin electrical connector and mine has three. The clerk told me that mine was a "Ford" model only. I'll google the numbers and see what's out there. I did hear a vacuum "hiss" when I removed the vacuum lines. I sucked on the bottom tube and felt the diaphragm pull. The second (longer) tube is clear and I can blow in or suck out without any restrictions. Hope it's supposed to be that way. Thanks again for any suggestions and I'll reply back with the outcome.
Bob

 

I think the part # for Ford is 7C3Z9H465A

SilverStateFordParts.com has it for $49.45 + S&H. MSRP is $69.97

 

Hi Guys,
I checked resistance measurements on my solenoid and found roughly 50 ohms between the three input terminals, one measuring 100 ohms. So, it appears that there are no opens or shorts in it. I checked the vacuum lines to the front wheel hubs and with only 18k miles on the truck, they all felt & looked good. No dry rotting. I reconnected the system and without starting the truck, had my wife power up the ventilation system, starting with the "vent" position. The first thing I heard was the vacuum pump start running. I also could feel it run and charge the vacuum reservoir. It ran for about a minute and then the pump stopped. I listened for any leaks around the reservoir or the front wheels and heard nothing. We went through all the ventilation positions and they all worked. I then switched into 4WD high and heard nothing. Then, we started the truck and again, tried all the vent positions & switching between 2WD & 4WD. All seems to work OK right now. Not sure what's going on but will keep an eye on it. Thanks to all that helped me understand the system and how to troubleshoot it. I think that there is still something that intermittently fails. I am going to cycle the system more before winter to hopefully find it.
Bob

 

you might have the same problem I do on my 99 model.
Somewhere electrical connector is not giving the power to the vacuum pump and the vent system go to the default what is defrost mode. Not having electrical schematics I never attempted to open the dash for troubleshooting, but hooked up jump wire to pump plug and when it does I jump it directly from the battery. The pump has build in pressure switch that will switch it off at preset vacuum. My problem was intermittent and didn't happen for a month.

 

Thanks for the info. Re the jump wire you mention - Does your jumper have a similar three conductor connector that plugs into the solenoid? What terminal do you apply the 12 vdc to? I labeled mine pin 1 {left side}, pin 2 {center}, and pin 3 {right side}. Pin 3 has a red conductor going to it so it may be the switched 12vdc. Thanks again for your help.
Bob

 

Hey Everyone,

I had the same problem with my F250. My system had a few problems in it. First my vacuum pump did not always turn back on when the vacuum was low or 0. It would get stuck off. The vacuum pump case actually opened and inside was a small pressure switch that had a set screw on it. I adjusted it a 1/4 turn and it was all it took to make it work like new! Very happy to have just saved $100 for a new pump.

I accidentially shorted the power wires for the pump and after some trial and error I found that on my 2004 Ford F250 the fuse #27 was the one that powers the turn signals and vacuum pump! NOT IN THE BOOK ANYWHERE!!!!!!!!

Now that the pump was working good I bypassed the solenoid and verified my hubs and lines down to them were good and holding pressure. When I put the solenoid back in the system it immediately lost vacuum and the pump would run continuously. The solenoid has a removable cover also. Underneath was a foam piece with two holes. One hole had a small round piece of rubber, the other was wide open. The only thing I can think of was this has been broke a long time or was not fixed right by the previous owner. If I plug the two holes with my fingers the solenoid works just fine. I may try to fabricate a new round rubber piece to create a seal when the vaccum is on it. If I can't then I think I just need a new solenoid and should be up and running.

 

I have the same intermittent problem on my 2005 F250. Frustrating! Hubs lock fine but AC defaults to defrost 75% of the time. Inside the solenoid is only 1 rubber piece. We have similar covers on the outlet of our gas masks to flow out only. It sucks in on itself when air goes in the opposite direction. Any way I think its the vacuum solenoid (Chiltons calls it the pulse vacuum solenoid) but I cant find one anywhere.

 

I think you are correct, it is the valve. On the '05 and up, with the new style hubs, my observation is that it is much less likely to be a problem down at the axle because they are sealed better. Other than a hose leak or bad pump, that valve is usually the problem.

I replaced my about 2 months ago. Ordered from PartsGuyEd, just before he went out on sick leave.

Part number is 7C3Z9H465A. It should be easy to find at any of the online Ford dealers. Cost was $55 plus shipping. Five minutes to change.

 

Thanks Bill. I'll try PartsGuyEd. I found it on ebay for $74. I have a feeling that the problem may be with the rubber diaphragm inside the solenoid but me in my infinite wisdom (ha ha) blew on it after I opened it up to clean out any small debris and it went flying into some unforseen and unreachable place. If it isn't seated properly the airflow may not go in the right direction. the AC defaults to defrost intermettintly even without that little diaphragm.

Greg

 

should the pump run constantly? even after I shut the truck down, and pull the lines there is still air in the tank 20 even 30 minutes later. why wont the pump shut down? 4x4, the heater, and ac works fine. WTF?

 

Doesn't sound right. My pump only runs when the ignition is ON. And by "air in the tank", I assume you mean vacuum?

How long does the pump keep running after you turn the truck off? Forever? The pump has an internal limit switch, and should turn itself off. But if it runs even with the truck shut off, that sounds like a bad relay somewhere that is stuck closed.